Development of Epitaxial AlN/GaN/GaInN Photocathode Heterostructures

for UV/Blue Scintillation and Cherenkov Radiation Detection

D.J. Leopolda,b,   J. Buckleya,   W. R. Binnsa,  P. Hinka, and  M.H. Israela  

(a)     Department of Physics and McDonnell Center for the Space Sciences Washington University, St. Louis, MO   63130
(b)Center for Molecular Electronics, University of Missouri, St. Louis, MO   63121


A need for extending the sensitivity of photon detectors to the blue and UV wavebands comes from the fact that both Cherenkov light and scintillation light typically have an emission spectrum that is peaked at short wavelengths. Photocathode layers consisting of wide-band-gap AlN/GaN/GaInN heterostructures are being compositionally tailored on an atomic scale during the epitaxial crystal growth process to control photoelectron absorption, diffusion, and transport to the negative electron affinity cathode surface.  These nitride alloy heterostructure layers are being grown directly on transparent single crystal sapphire substrates in ultra-high vacuum by molecular beam epitaxy.  This technology is expected to significantly enhance UV/blue radiation detection sensitivity in single photon counting imaging and non-imaging devices.

Direct space-based imaging of rapid UV/blue astrophysical transients will greatly benefit from photon counting detectors with high quantum efficiency. Also X-ray, Gamma-ray, and cosmic-ray experiments that employ scintillation light detectors or Cherenkov detectors would benefit greatly from photomultipliers with higher quantum efficiencies. In this case the need for increasing the sensitivity of photon detectors in the blue and UV wavebands comes from the fact that both Cherenkov light and scintillation light have an emission spectrum that is peaked at short wavelengths. Photomultiplier tubes in use today for high-energy particle detection applications have a significant spectral mismatch with typical sources.  This point is demonstrated in Fig. 1 , where we have displayed a typical Cherenkov light spectrum as well as the emission from a plastic scintillator together with the detection sensitivity curve of a bialkali photomultiplier tube and a tube incorporating a state-of-the-art GaAsP photocathode. The short wavelength response of the GaAsP tube shown in Fig. 1 has been enhanced through the use of a wavelength shifter coating on the window [1].

Photomultiplier tubes are high-gain optical detectors capable of photon detection over a wide spectral range.   The heart of such a device is the photocathode.   By the photoelectric effect, a photon incident on the photocathode ejects an electron which then is accelerated by an electrical potential to produce a number of secondary electrons either through a cascade of interactions in a dynode chain or through a direct interaction with a photodiode.  These secondary electrons constitute the measured signal. The quantum efficiency of such a device is defined in terms of an optical to electrical conversion percentage, determined by the number density of initial photoelectrons produced per incident photon flux.

Figure 1.






Our research on high quantum efficiency photocathodes involves the design and fabrication of precisely tailored heteroepitaxial semiconductor structures that have peak sensitivity in the UV/blue spectral range.  After considering the optoelecronic and structural properties of different wide-band-gap semiconductor alloy materials we have determined AlN/GaN/GaInN to be the most suitable candidate.  The band gap of this system can be tailored over an energy range from 1.9 to 6.2 eV and epitaxial thin film layers can be grown directly on optically transparent sapphire substrates [2]. GaAlN/GaInN has already been recognized as a leading material in the fabrication of wide-band-gap laser diode devices, making it a logical choice for heteroepitaxial photocathode development.   The AlN/GaN/GaInN heterostructures discussed in this work have been fabricated in ultra-high vacuum by molecular beam epitaxy (MBE). The use of MBE for crystal growth makes it possible to control film composition on an atomic scale and to fabricate abrupt heteroepitaxial interfaces.   The ultra-high vacuum conditions and cryogenically cooled chamber walls allow for very quick on/off switching of atomic and molecular beams through the use of shutters in front of each thermal or electron beam source.   A reflection high energy electron diffraction (RHEED) system mounted inside the vacuum chamber allows the surface crystal quality to be monitored and individual atomic layers to be counted during growth as they are added to the surface one at a time by examining surface reconstruction diffraction patterns. This feedback provides the absolute finest control of the heteroepitaxial crystal growth process, thereby making possible precise fabrication of semiconductor layered structures for use in high performance devices

All AlN/GaN/GaInN heterostructures used in these studies are grown on single-crystal sapphire substrates. The mechanical strength and UV/visible optical transparency properties of sapphire make it an excellent choice as a window material for photocathode structures. In order to increase the quantum efficiency of AlN/GaN/GaInN photocathodes a couple of key design features are incorporated in our heteroepitaxial layers.   Examples of this are shown in Fig. 2 , where conduction and valence band edge energy spatial profiles are displayed for two possible photocathode designs. This diagram is basically a plot of band gap versus depth into the photocathode structure.  For clarity the individual layer thicknesses shown in Fig. 2 are not drawn to scale but instead should be regarded as a rough schematic to illustrate the main design features.  An AlN optical antireflection layer inserted between the sapphire and the GaInN photocathode region serves as a wide-band-gap barrier to prevent electronic back diffusion into the substrate interfacial region where defect densities are expected to be higher and nonradiative recombination of photoexcited electrons larger.  Inserting this wide band gap AlN buffer layer in the structure ensures that photoexcited electrons do not diffuse back toward the sapphire substrate interface, but instead are reflected toward the photocathode emission surface.
Figure 2.

It is known that an electric field applied inside a semiconductor photocathode layer drives electrons toward the emitting surface and in so doing can increase the quantum efficiency by as much as a factor of two [3].  As shown at the top of Fig. 2 we plan to create these internal fields in the photocathode layer by grading the alloy composition, which tilts the conduction and valence band edges Ec and Ev. As the In concentration in the layer is increased the energy gap between valence and conduction band decreases, resulting in a sloping of the band edges. Although the fractional  amount   of  change  in the conduction and valence bands are different, the overall effect is to tilt the conduction band since the p-type dopant incorporated throughout the layer allows mobile hole charge carriers to diffuse in a manner that minimizes the energy, leaving the valence band profile Ev flat.  The tilted conduction band drives photoexcited electrons toward the surface, increasing their escape probability and thus the quantum efficiency. Finally, an activation layer of CsO on the surface bends the bands to achieve a negative electron affinity (NEA) condition, which is vital for having a high photoelectron escape probability. We are also investigating novel means of activating the photocathode emitting surface in order to achieve the negative electron affinity condition.  Since AlN and GaAlN with high Al concentration have an intrinsic negative electron affinity surface we are examining the possibility of ending the epitaxial layers with a Si-doped GaAlN layer in order to achieve an NEA surface without the need for post-growth CsO activation. This design is shown at the bottom of Fig. 2.   By including these design features in layers made with wide band gap AlN/GaN/GaInN semiconductor materials we expect to increase the photocathode quantum efficiency response in the UV/blue spectral range.

At the present time AlN/GaInN photocathodes have been designed and are being fabricated on up to 2-inch diameter sapphire substrates by MBE. The structural, optical, and electronic properties of these heterostructures are being evaluated. X-ray diffraction and TEM lattice image studies show good registry of epitaxial GaN and GaInN layers with the c-plane-oriented, single-crystal sapphire substrates. Optical absorption measurements of GaN and GaInN confirm the expected band gap shift with increasing indium concentration. Also, a photoelectric emission measurement stage is being designed for use in the ultra-high-vacuum MBE system. This stage will be used to measure the  quantum efficiency and spectral responsivity of as-grown AlN/GaInN photocathode heterostructures  without breaking vacuum.  Overall this new nitride-based photocathode technology is expected to have a significant impact on Cherenkov and scintillation radiation detection where emission is peaked in the UV/blue spectral range, and on direct space-based imaging of rapid UV/blue astrophysical transients.

This research is supported by NASA Grant # NAG5-8536 under the Explorer Technology Program.

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